Arsenic is a toxic element that naturally occurs in a variety of combined forms. Its presence in natural waters may originate, for example, from geochemical reactions, industrial waste discharges and past and present agricultural uses of arsenic-containing pesticides. Because the presence of arsenic may have carcinogenic and other deleterious effects on living organisms, the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO) have set the maximum contaminant level (MCL) for arsenic in drinking water at 10 parts per billion (ppb). Arsenic concentrations in wastewaters, ground waters, surface waters and geothermal waters frequently exceed this level. Thus, the current MCL and any future decreases, which may be to as low as 2 ppb, create the need for new techniques to economically and effectively remove arsenic from drinking water, well water and industrial waters or any other waters.
Arsenic occurs in four oxidation or valence states, i.e., −3, 0, +3, and +5. Under normal conditions arsenic is found dissolved in aqueous or aquatic systems in the +3 and +5 oxidation states, usually in the form of arsenite (AsO33−) and arsenate (AsO43−). The effective removal of arsenic by coagulation techniques requires the arsenic to be in the arsenate form. Arsenite, in which the arsenic exists in the +3 oxidation state, is only partially removed by adsorption and coagulation techniques.
Various technologies have been used in the past to remove arsenic from aqueous systems. Examples of such techniques include adsorption on high surface area materials, such as alumina and activated carbon, ion exchange with anion exchange resins, co-precipitation and electro-dialysis. However, most technologies for arsenic removal are hindered by the difficulty of removing arsenic at very low levels and maintain an effective loading capacity. Additionally the amount of material necessary to remove arsenic at low levels can make the technology economically unfeasible. The more successful techniques that have been used in large municipal water supplies are not practical for residential applications because of space requirements and the need to use dangerous chemicals. The two most common techniques for residential water treatment have been reverse osmosis and solid filtration media such as activated alumina, iron oxide, and titanium oxide. The former method produces arsenic-containing waste streams that must be disposed of, and the latter requires the media to treat sufficiently large volumes of water in order to be economically viable.
The above facts coupled with the potential for the decrease in MCL to 10 ppb or less make it imperative that effective sorbents and/or reactants to remove arsenic from water and other aqueous systems be developed and used.